This invention relates to a fuel system for an aircraft, and in particular to a fuel system having a low-pressure start-up capability.
A number of fuel systems incorporating a low pressure start-up capability are known. For example, U.S. Pat. No. 6,666,014, U.S. Pat. No. 6,321,527 and US 2005/0262824 all describe arrangements in which the pressure applied to a control chamber of a pressure raising and shut-off valve (PRSOV) is controllable by an external valve arrangement. In U.S. Pat. No. 6,666,014, the external valve arrangement is incorporated into a metering valve, the external valve arrangement being incorporated into a pressure drop control valve in U.S. Pat. No. 6,321,527 and into a combining spill valve in US 2005/0262824.
U.S. Pat. No. 6,176,076 provides a low pressure start-up capability by providing a passive restrictor in a spill return line.
The arrangements in which an external valve arrangement is provided are fairly complex, and the passive restrictor approach to achieving a low pressure start-up results in the fuel system operating at reduced efficiency when operating at high flow or pressure conditions as the restrictor gives rise to parasitic losses from the system.
It is an object of the invention to provide a fuel system having a low pressure start-up capability which is of simple and convenient form.